If its a myth, then why have I seen, and experienced people having different points of impact using the same sight settings?

Because we hold the gun differently at the moment of firing? Or something else?

The reason different people get different points of impact with the same sight settings is as I explained before; we all don't hold the firearm the exact same way. The firearm recoils in different amounts and in different directions while the bullet's going down the barrel.

Our eyes all will see the same sight picture with scopes as well as with open or aperture front and rear sights when they're aligned the same way on the target. The exact same thing happens when a digital camera mounted on a sturdy tripod and focused at some object and its perfectly centered on something. Everyone behind it looking at the image on the LCD display sees it the same regardless of where they are behind it. That image on the camera's sensor is done exactly the same way our eyes focus it on our retina; there ain't one bit of difference. Every one behind that camera will see exactly the same image and everything appears the same.

Those not believing this might consider asking their favorite eye doctor about it. If those professionals don't convince one otherwise, then maybe those disbelievers should explain why rays of light travel through sights differently for different people and what laws of physics change to make that happen depending on the person looking through the sights.

A good way to see just the differences in how one uses their trigger hand on a rifle is to mount a laser pointer on it adjusted to put the spot at the point of aim. Then have someone watch them dry fire the rifle and see what direction and how much it moves when the firing pin falls. Most folks are pretty surprised at how much they jerk the rifle off its point of aim from where it was just before the firing pin fell. Then see what happens when several people do this with the same rifle.